IB/mlx5: Allocate a Transport Domain for each ucontext

Transport Domain groups several TIS and TIR object. By grouping
these object, it defines wheather local loopback packets that
are sent from the TIS objects in the group are received by the
TIR objects in the same group.

Allocate a Transport Domain(TD) for each user context to be used
in the future by Raw Packet QP for Self-Loopback Control.

Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>